Structural arrangements for the opened circuit liquid cooling of gas turbine buckets are shown in Kydd U.S. Pat. Nos. 3,445,481 and 3,446,482. The former patent discloses a bucket having cooling passages open at both end which are defined by a series of ribs forming part of the core of the bucket and a sheet metal skin covering the core and welded to the ribs. The latter patent teaches squirting liquid under pressure into hollow forged or cast gas turbine buckets. A further Kydd patent, U.S. Pat. No. 3,619,076 describes a system where a turbine blade construction involves a central air foil shaped spar which is clad with a sheet of metal having very high thermal conductivity and grooves recessed in the sheet face adjacent to the spar, which grooves together with the smooth surface of the spar define coolant passages distributed over the surface of the turbine blade. Bonding a sheet to a core in such configurations involve numerous disadvantages. When a braze is used to bond the skin, some channels of the turbine buckets become plugged and obstructed with the braze material. Further, excellent bonds are required between the core and skin to contain the water in full channel flow under the extremely high hydraulic pressures which result from the centrifugal forces during operation of the turbine. Any cracks in the skin can cause leakage of the coolant and result in vane failure.
Many of the disadvantages of such prior art are overcome by the invention disclosed in Anderson U.S. Pat. No. 4,156,582 in which water cooling channels are formed using preformed tubes located beneath an outer protective layer composed of an inner skin to provide a high thermal conductivity and an outer skin to provide protection from hot corrosion.
Schilling, et al U.S. Pat. Nos. 3,928,901 and 3,952,939 disclose methods of attaching a sheet cladding to a convex-concave substrate such as an air foil or a turbine bucket using hot isostatic pressing techniques. These procedures, however, when applied to the manufacture of turbine buckets incorporating preformed tubes will tend to collapse the tubes. Additionally, when molten glass is used as the pressure transmitting medium as disclosed in the U.S. Pat. No. 3,952,939 patent, the molten glass can enter the tubes and is difficult or almost impossible to remove without damage to those tubes. To overcome such problems Schilling et al in U.S. Pat. No. 4,183,456 extend the ends of the tubing above the molten glass so that the pressure inside and outside the tubing is maintained at equilibrium to prevent tube collapse during the application of the hot isostatic pressure.
It is desirable to cause turbulence of the cooling fluid as it traverses the cooling passages in cast or wrought gas turbine nozzles or buckets. Such turbulence increases the heat flux passing from the hotter gas path surfaces to the cooler internal fluid which is being used to remove the heat. At the same time, it is desirable to eliminate surface roughness of the cooling passages in order to improve the fatigue life of substrate containing the cooling passages.
A review of the theory and techniques to increase heat transfer can be found in an article by Arthur E. Bergles entitled "Survey and Evaluation of Techniques to Augment Convection Heat and Mass Transfer" which begins at page 331 in volume 1 of Progress in Heat and Mass Transfer, edited by U. Grigull and E. Hahne (Pergamon Press, 1969). As pointed out therein, surface promoters including surfaces obtained by machining or protuberances of the insert variety such as wire coils inserted inside tubes are known. Turbine blades having longitudinally extending internal passages for the flow of cooling fluids are shown in British Pat. Nos. 651,830 and 728,834. In the former patent, a finned insert formed separately from the blade is placed in the passage with the fins in good heat conducting relationship with the wall of the passage and in the latter patent, the fin pitch is varied along at least a part of the length of the passageway and the rod insert itself may be removed leaving a fin with a hollow core. A configuration in which a plurality of elements are positioned within the passageways extending traversely with respect thereto and being apertured to permit flow of fluid is shown in U.S. Pat. No. 2,843,354.
Liquid cooled buckets in which cooling passages which are cylindrical in configuration are described in U.S. Pat. Nos. 4,098,010, 4,119,390 and 4,142,831. In the first named patent, cylindrically shaped coolant passages for liquid cooled turbine buckets are converted into at least two helical subpassageways by flow splitting means introduced into individual coolant passages and fixed in place as by brazing or tight mechanical fit. In addition, each flow splitting or flow modifying means is provided with means disposed therealong for interupting the liquid flow in each helical subpassageway. The second patent describes a plurality of oriented spanning elements which are affixed in and extend across each cooling passage. The last named patent, U.S. Pat. No. 4,142,831, provides each of the individual coolant passageways in the airfoil portion of a liquid cooled turbine bucket with a plurality of circumferentially extending crimps or protrusions, located at spaced intervals along each coolant passage, each protrusion extending along the inner periphery of the coolant passage over ancuate length of at least about 120.degree. being disposed in a plane generally perpendicular to the wall of the coolant passage at that location. The flow of liquid coolant moving in each of such coolant passages during the operation of the turbine under the influence of centrifugal forces is broken up and dispersed upon encountering the protrusions thereby contacting a larger area of the interior of the coolant passage.